The present invention relates to a developing device, and more particularly to a developing device in an electrophotographic recording apparatus.
Generally, in an electrophotographic recording apparatus, a photosensitive body, the surface of which is uniformly charged, is irradiated with light to form an electrostatic latent image, and toner is deposited on the electrostatic latent image portion by an electrostatic force to produce a visible image, which is transferred to a printing medium, and fixed by heating the toner on the printing medium.
FIG. 11 is a block diagram of a developing device of reversal development type in a conventional electrophotographic recording apparatus. The device in FIG. 11 comprises a photosensitive drum 11, a charging roller 12, a print head 13, a developing device 14, and a transfer roller 15.
The photosensitive drum 11 is a cylindrical photosensitive body, which is a working part to have an electrostatic latent image formed by the charging roller 12 and the print head 13, have toner from the developing device 14 deposited on the latent image, and get the toner transferred to a printing medium 16. The charging roller 12 is a cylindrical roller set face to face with the photosensitive drum 11 to uniformly electrify the surface of the photosensitive drum 11. The print head 13 is arranged downstream of the charging roller 12 in the rotating direction of the photosensitive drum 11 to form an electrostatic latent image by irradiating the photosensitive drum 11 with light for exposure.
The developing device 14 to develop the electrostatic image to produce a toner image is arranged downstream of the print head 13 in the rotating direction of the photosensitive drum 11, and comprises a developing roller 21, sponge roller 22, toner 23, an agitator rod 24, a developer scraping blade 25, and a case 26. The case 26 contains and holds the toner 23.
The developing roller 21 is a cylindrical roller, which rotates in the arrow direction 21a, is arranged face to face with the photosensitive drum 11 to deposit toner on the electrostatic latent image formed on the photosensitive drum 11. The sponge roller 22 is in contact with the developing roller 21 and rotates in the same direction (in the arrow direction 22a) as the developing roller 21. After the toner is deposited on the photosensitive drum 11, the sponge roller 22 removes surplus toner from the developing roller 21, and, on the other hand, supplies toner afresh to the developing roller 21. The sponge roller 22 has at its center a conductive rod 22b to which a voltage Vs is applied.
The toner 23 is developing powder which is deposited on the electrostatic latent image on the photosensitive drum 11 to develop the image. The agitator rod 24 agitates the toner 23 in the case 26 to homogenize it. The developer scraping blade 25 thins down the toner 23 deposited on the surface of the developing roller 21 to a thin film of a constant thickness.
The transfer roller 15 rotates in the direction of the arrow and is arranged downstream of the developing device 14 in the rotating direction of the photosensitive drum 11 and is pressed to the photosensitive drum 11. The transfer roller 15 has a toner image transferred to the printing medium 16 fed between the photosensitive drum 11 and the transfer roller 15.
In the electrophotographic recording apparatus configured as described, when a motor, not shown, rotates to feed the printing medium 16 in the direction of the arrow 16a, the developing roller 21 and the sponge roller 22 in the developing device 14 rotate in the directions of arrows 21a and 22a respectively, and the toner 23 in the developing device 14 is agitated by the agitator rod 24 which rotates in the direction of the arrow 24a. At this time, negative voltages are applied to the developing roller 21 and the sponge roller 22 simultaneously, but in such a way that the absolute value of the applied negative voltage of the developing roller 21 is smaller. In the case of reversal development, negatively-charged toner 23 that was agitated is deposited on the surface of the sponge roller 22, and the deposited toner 23 is supplied to the surface of the developing roller 21 through contact of the sponge roller 22 with the surface of the developing roller 21 having a negative voltage the absolute value of which is smaller than that of the sponge roller 22. The toner deposited on the surface of the developing roller 21 is made thinner by the developer scraping blade 25 to a thin toner layer.
On the other hand, simultaneously with the rotation of the above-mentioned motor, not shown, a negative voltage is applied to the charging roller 12, and the photosensitive drum 11 is charged uniformly while rotating in the direction of the arrow 11a. After this, when the photosensitive drum 11 is irradiated with light from the print head 13, the regions exposed to light decrease in the level of charge to 0[V], so that an electrostatic latent image is formed on the surface of the photosensitive drum 11. The toner 23 deposited on the developing roller 21 is negatively-charged, and when the developing roller 21 contacts the electrostatic latent image on the photosensitive drum 11 or reaches a maximum proximate point, the toner 23 is deposited on the exposed regions. Meanwhile, because the regions not exposed to light are more negatively charged than the negatively-charged toner 23 on the developing roller 21, the toner 23 is not deposited on the unexposed regions. Subsequently, the toner image is transferred from the photosensitive drum 11 to a printing medium 16, and the toner image is fixed to the printing medium 16 by a fixing unit, not shown.
However, the conventional developing device has problems as follows.
FIG. 12 is a diagram for explaining the conventional problem.
The residues of toner (residual toner) 101 on the developing roller 21, which was not used by the photosensitive drum 11 and returned to the developing device 14, are scraped off on the downstream side of the contact area by the sponge roller 22. However, in a continuous printing process, for example, the residual toner 101 gets into a sponge portion 22c (indicated as toner 102) covering the periphery of the sponge roller 22, and solidifies to give rise to clogging of the sponge, thus deteriorating the toner removing performance of the sponge roller 22. Under this condition, the residual toner 101 is not detached completely on the downstream side of the contact area, and stays deposited on the developing roller 21 as indicated by the residual toner 101a. Consequently, there arises a difference in toner thickness on the surface of the developing roller 21 or a difference in potential of the toner between its first rotation and the second rotation when the residual toner 101a remains deposited on the developing roller 21, which is responsible for irregular print density.
In solid print, the image, which was used for printing in the first rotation, remains as a thin residual image in the second rotation. In other words, toner is deposited on the previous toner image which was not consumed on the photosensitive drum 11 and remains on the developing roller 21, with the result that a reverse image of the image in the first rotation is printed.
Further, irregular print density occurs for the reason as follows. The residual toner, which was once detached by scraping off from the developing roller 21, still stays deposited on the sponge roller 22. As the sponge roller 22 rotates, the residual toner is supplied together with other toner to the surface of the developing roller 21, making the toner layer not uniform and giving rise to irregular print density.
Irregular print density occurs for another reason. Immediately before starting printing, the image forming process starts, and to this end, the photosensitive drum 11 is subjected to preliminary charging by the charging roller 12. Simultaneously with the charging of the photosensitive drum 11, the surface of the developing roller 21 is also charged. Therefore, in a printing process with high density which consumes a relatively large amount of toner, such as solid print, a difference occurs in toner layer thickness between the first rotation of the developing roller 21 and in the second rotation when the surface is reset by detaching the toner once.
As mentioned above, the problem with the prior art is that irregular print density occurs which is attributable to the unevenness of the thickness of the toner layer.
A developing device for supplying, by a developer carrying body, a developer to an electrostatic latent image formed on a photosensitive body is proposed. The device includes a developer supplying member to supply a developer to the developer carrying body and to separate the developer remaining on the developer carrying body. Developer removing means is provided for contacting the developer supplying member and for removing the developer adhering to the developer supplying member.